Problem 12 · 2020 Math Kangaroo
Medium
Logic & Word Problems
place-valuesymmetry
When the bat Elisa left her cave at night, the digital clock showed the time on the left. When she came back in the morning and hung herself upside down, she read her watch and saw the time on the right. How long did she stay out of the cave?
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Answer: D
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Hint 1 of 2
The morning time is read on an upside-down watch, so first turn that reading the right way up before comparing it with the night-time clock.
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Hint 2 of 2
Each digit, turned a half-turn, becomes another digit (0,1,2,5,8 still read as digits) and the whole reading flips left-to-right; fix the morning time, then count the gap.
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Approach: turn the upside-down morning reading right way up, then count the gap
- The night clock shows the leaving time directly.
- The morning watch is held upside down, so rotate that display a half-turn: each digit changes shape and the left and right digits swap, giving the true morning time.
- Counting forward from the night time to the corrected morning time gives the time she was away, 3h 41m.
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